The present invention relates to a method for increasing the selectivity to light olefins and/or for decreasing C4+ formation in oxygenate conversion reactions.
Light olefins, defined herein as ethylene, propylene, and mixtures thereof, serve as feedstocks for the production of numerous important chemicals and polymers. Heavy olefins are defined herein as the hydrocarbon products heavier than light olefins. Light olefins traditionally are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies, based on alternative feedstocks have increased.
Important types of alternate feedstocks for the production of light olefins are oxygenates, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The reaction, which converts oxygenates to desired light olefins, also produces by-products. Representative by-products include, for example, alkanes (methane, ethane, propane, and larger), C4+ olefins, aromatic compounds, carbon oxides and carbonaceous deposits (also referred to as xe2x80x9ccokexe2x80x9d).
During the conversion of oxygenates to light olefins, carbonaceous deposits accumulate on the catalysts used to promote the conversion reaction. As the amount of these carbonaceous deposits increases, the catalyst begins to lose activity and, consequently, less of the feedstock is converted to the light olefin products. At some point, the build up of these carbonaceous deposits causes the catalyst to reduce its capability to convert the oxygenates to light olefins. When a catalyst can no longer convert oxygenates to olefin products, the catalyst is considered to be deactivated. Once a catalyst becomes deactivated, it must be removed from the reaction vessel and replaced with fresh catalyst. Such complete replacement of the deactivated catalyst is expensive and time consuming. To reduce catalyst costs, the carbonaceous deposits are periodically fully or partially removed from the deactivated and/or partially deactivated catalyst to allow for reuse of the catalyst. Removal of the deactivated catalyst and/or partially deactivated catalyst from the reaction process stream to remove the carbonaceous deposits is typically referred to as regeneration and is typically conducted in a unit called a regenerator.
Previously in the art, catalyst regeneration was accomplished by removing the deactivated catalyst from the process stream, removing the carbonaceous deposits from the catalyst, and then returning the regenerated catalyst to the reactor near the inlet of the reactor or reaction vessel. Conventionally, this inlet is located near the bottom quarter of the reactor or reaction vessel. By returning the regenerated catalyst near the inlet of the reactor, the regenerated catalyst would immediately contact fresh feedstock and begin conversion of the feedstock. However, doing so does nothing to control the conversion of the feedstock into by-products.
For example, U.S. Pat. No. 4,873,390 to Lewis et al. teaches a process for catalytically converting a feedstock into a product in which the feedstock is contacted with a partially regenerated catalyst. Lewis et al. describes that a partially regenerated catalyst improves the selectivity of the process to the light olefin products. While contacting the feedstock with a partially regenerated catalyst may improve the selectivity of the process to the light olefin products, it does nothing to control production of by-products.
For these reasons, a need exists in the art for improved processes which increase light olefin selectivity and control production of by-products.
The present invention solves the current needs in the art by providing a method for increasing light olefin production and controlling production of by-products.
One aspect of the present invention is directed to a method for selectivating a catalyst. As used herein, the word xe2x80x9cselectivatexe2x80x9d (or selectivating) refers to a process by which a certain amount of carbonaceous deposits are formed on the catalyst to cause the catalyst to produce more ethylene and propylene from the oxygenate feed and to produce fewer by-products. In the present invention, the selectivation of the catalyst occurs by contacting the catalyst with the by-products of the conversion reaction. However, the light olefins produced by the oxygenate conversion reaction can also be used for the selectivation of the catalyst, either separately or in combination with the by-products. As one well skilled in the art will appreciate, it is preferred that as much as possible of the by-products selectivate the catalyst and as little as possible of the light olefins selectivate the catalyst. As the by-products contact an at least partially regenerated catalyst, primarily the C4+ olefin portion of the by-products, is primarily converted to light olefins and carbonaceous deposits which form on the catalyst. While the accumulation of the carbonaceous deposits does contribute to the deactivation of the catalyst, the accumulation of the carbonaceous deposits also contributes to the selectivation of the catalyst. When the selectivated catalyst mixture then contacts the oxygenate feed, selectivity of the conversion reaction to forming light olefins, particularly ethylene and propylene, is increased and/or formation of by-products is reduced compared to using a fresh or regenerated catalyst that has not been selectivated according to the present invention.
The method for selectivating the catalyst comprises contacting, in a reactor, a feedstock, including oxygenates, with a molecular sieve catalyst under conditions effective to form a product including light olefins and by-products, the contacting causing carbonaceous deposits to form on at least a portion of the molecular sieve catalyst producing deactivated catalyst; removing a portion of the deactivated catalyst from the reactor; regenerating the portion of the deactivated catalyst to remove at least a portion of the carbonaceous deposits from the deactivated catalyst removed from the reactor to form an at least partially regenerated catalyst; and exposing at least a portion of the at least partially regenerated catalyst to at least a portion of the by-products to selectivate the at least partially regenerated catalyst to form light olefins. In this process, the at least partially regenerated catalyst may also be exposed to at least a portion of the light olefins to selectivate the at least partially regenerated catalyst to forming light olefins. This process may also include the step of contacting at least a portion of the selectivated at least partially regenerated catalyst with the feedstock.
Another aspect of the present invention is directed to a method for converting oxygenates to light olefins. The method comprises contacting, in a reactor, a feedstock comprising oxygenates with a molecular sieve catalyst under conditions effective to convert the feedstock to a product including light olefins and by-products, the contacting causing carbonaceous deposits to form on at least a portion of the molecular sieve catalyst producing deactivated catalyst; removing a portion of the deactivated catalyst from the reactor; regenerating the portion of the deactivated catalyst under conditions effective to remove at least a portion of the carbonaceous deposits from the deactivated catalyst to form an at least partially regenerated catalyst; exposing at least a portion of the at least partially regenerated catalyst to at least a portion of the by-products to selectivate the portion of the at least partially regenerated catalyst to form light olefins; and contacting the selectivated portion of the at least partially regenerated catalyst with the feedstock to form the product. In this method, the at least partially regenerated catalyst may also be is exposed to at least a portion of the light olefins to selectivate the at least partially regenerated catalyst to forming light olefins. This method may include the additional step of recovering the light olefins. If the light olefins are recovered, then this method may also include the step of polymerizing the light olefins to form polyolefins.
Still another aspect of the present invention is directed to a method for reducing the heat of reaction in a reactor by offsetting the exothermic conversion of a feedstock during a catalyzed chemical conversion process. The method comprises contacting, in a reactor, a feedstock with a catalyst under conditions effective to form a product and by-products, the contacting causing carbonaceous deposits to form on at least a portion of the catalyst causing at least a portion of the catalyst to become deactivated catalyst; removing at least a portion the deactivated catalyst from the reactor; regenerating the portion of the deactivated catalyst removed from the reactor to remove at least a portion of the carbonaceous deposits from the deactivated catalyst to form an at least partially regenerated catalyst; and contacting the at least partially regenerated catalyst with the by-products to facilitate an endothermic reaction with the by-products.
Yet another aspect of this invention provides a method for converting heavy olefins present in a product stream into light olefins and carbonaceous deposits on a catalyst without separation of the heavy olefins from the product stream exiting the first reaction zone. The method comprises the following steps: producing a product stream in a first reaction zone, the product stream including heavy olefins; moving the product stream from the first reaction zone to a second reaction zone without separation of the heavy olefins from the product stream; and contacting the product stream with a catalyst in the second reaction zone under conditions effective to form the light olefins, the contacting causing carbonaceous deposits to form on at least a portion of the catalyst producing a deactivated catalyst.
Another aspect of this invention provides a method for controlling the conversion of heavy olefins into light olefins. This method comprises the following steps: creating a first product stream in a first reaction zone, the first product stream comprising heavy olefins; moving the first product stream to a second reaction zone; and contacting the first product stream with a catalyst present in the second reaction zone under conditions effective to form a second product stream comprising the light olefins, the contacting causing carbonaceous deposits to form on at least a portion of the catalyst producing a deactivated catalyst, wherein the amount of the catalyst in the second reaction zone is controlled by a mechanism. The first product stream can be the same or different from the product stream exiting the first reaction zone.
Other advantages and uses of the process of the present invention will become apparent from the following detailed description and appended drawing and claims.